1. Field of the Invention
The present invention relates to a dimmer triggering circuit for low-load applications, e.g. LED-based light sources. The invention further relates to a dimmer system comprising such a dimmer triggering circuit.
2. Description of the Related Art
Generally, phase-controlled dimmers comprise a TRIode for Alternating Current, further referred to as a TRIAC. A TRIAC is a bidirectional switch which can conduct current in either direction when it is triggered, i.e. turned on. It can be triggered by either a positive or a negative voltage being applied to its gate electrode, i.e. when a small current is applied to its gate. This current only needs to be applied for a short period of time, i.e. in the order of microseconds. In other words, the TRIAC needs to be triggered or ‘fired’. Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, such as at the end of a half-cycle of alternating current (AC) mains power, also referred to as a zero-crossing. As a result, the TRIAC then ‘turns off’.
The aforementioned dimmers work very well for dimming incandescent bulbs. It is widely recognized that dimmers do not perform properly if they are used in low-load dimmer applications, e.g. light sources based on a light-emitting diode (LED). LEDs consume little power, and are therefore unable to operate the TRIAC-based dimmer as intended.
International application WO2005/115058 describes a dimmer system comprising a dimmer connected to a dynamic dummy load to allow a phase control dimmer to be used with LED lighting. The dynamic dummy load provides a load to the dimmer when the LEDs do not provide sufficient load. On the other hand, it provides a reduced flow of current when the LEDs provide sufficient current draw from the dimmer. However, in order to enable the dimmer system to work properly, the dynamic dimming load is required to keep the current through the TRIAC in the dimmer above its holding current. Furthermore, a dynamic load control signal is needed, which makes the dimmer system complex and difficult to implement. Finally, the dynamic dimming load described in WO2005/115058 draws a current of several mA, even when not active, resulting in considerable waste of energy.
U.S. Pat. No. 7,102,902 describes a dimmer system for dimming LEDs utilizing a dimmer comprising a TRIAC. The load applied to the dimmer is controlled such that it supplies a resistive load when needed and does not otherwise. However, the circuit described must be customized to the minimum load of the dimmer. Depending on the type of dimmer used, some components need to be adapted, which makes the dimmer system inflexible. The circuit also relies on adding a large load to the system, resulting in high current components being required in the circuit and high power loss in the system.
It is an object of the invention to provide a dimmer triggering circuit for triggering a dimmer which is unsuitable to dim LEDs otherwise, the dimmer circuit being suitable for use in combination with a wide variety of dimmers designed for incandescent bulbs, including halogen lamps, while consuming limited power. The circuit may also be used in other types of circuits where a minimum power below the specified minimum load of the dimmer is required.